This invention relates to the field of fiber optic communication, and more specifically to a method and an apparatus that provides mode multiplexing and holographic demultiplexing in the field of multimode fiber optic communication systems.
The present invention provides for the multiplexing of a number of information channels on a multimode optical fiber; thereby allowing more information to be transmitted down the fiber at a relatively low cost.
In brief, there are two general classes of optical fibers, single mode fibers and multimode fibers. Multimode fibers are generally less costly that single mode fibers, as are the input/output mechanisms that are used to inject light into the input of the fibers and to receive light from the output of the fibers.
A single mode fiber is generally of a very small diameter. Thus, all components associated with the input/output thereof are physically small and must be manufactured with high precision. By definition, single mode fibers have but one spatial channel having a large frequency bandwidth.
While multimode fibers have a cost advantage, the art has generally been unable to utilize the many communication channels or modes of these fibers, so that usually one channel of limited band width is used. One of the problems with using the many channels of a multimode fiber is the problem of demultiplexing the fiber's multi channel output.
Examples of arrangements generally useful in the practice of the present invention include U.S. Pat. No. 4,770,485 which describes a mechanical apparatus that is useful for launching separate beams into an optical fiber as first and second mode groups.
Examples of the use of multimode fibers in a communication system include, (1) the publication OPTICS LETTERS, Optical Society of America, November 1980, Vol. 5, No. 11, pages 485-487, which describes apparatus including a multimode fiber wherein the acoustically induced phase modulation of coherent light that is propagated by the fiber is detected by the use of an interferometer which employs an unmodulated replica of the fiber's output beam as a reference. A dynamic, adaptive, hologram provides this reference. More specifically, a hologram of the wave front emanating from the fiber at one time is interfered with the fiber wave front at another time; (2) the publication JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 8, No. 7, July 1990, at pages 1039-1045, and at pages 1123-1126, which describes arrangements for measuring the correlation between the speckle radiation pattern of a multimode fiber and a reference pattern that is stored in a holographic matched filter. More specifically, a hologram is made of the output of the multimode fiber in order to record an initial state of the fiber. This matched filter hologram is then used as a reference against which changes in the fiber's output, due to the influence of axial strain and the like, is compared; and (3) U.S. Pat. No. 3,761,716 which describes an optical waveguide mode discriminator that provides a plurality of electrical signals, each signal corresponding to the information that is propagated by one of a plurality of optical modes of a multi mode optical waveguide. The output light beam of the multi mode optical waveguide, i.e. the beam containing the plurality of optical modes, is then processed by a plurality of mode discriminator optical waveguides which operate to selectively transmit only certain of the optical modes. The output beams of these mode discriminator optical waveguides are then detected, and the resulting electrical signals are processed in a manner to obtain one electrical signal for each of the optical modes. This patent also cites U.S. Pat. No. 3,157,726 for its teachings relative to the use of the optical and physical properties of an optical fiber, such that the separate modes thereof can be used as channels for the independent but simultaneous transmission of different information on each channel, and cites U.S. Pat. No. 3,711,267 as an exemplary multi mode optical waveguide for use therein.
Examples of fiber optic arrangements providing mode multiplexing include, (1) the publication APPLIED OPTICS, Vol. 21, No. 11, 1 Jun. 1982, at pages 1950-1955, which describes apparatus having mode division multiplexing, the apparatus including a multimode graded index optical fiber and a bimodal spatial mode filter which is located at the input of the optical fiber; and (2) U.S. Pat. No. 4,062,618 which describes a spatially multiplexed optical communication system wherein demodulation is accomplished by the use of holographic matched filtering. In this system a holographic system 1 receives a plurality of intelligence signals S1-Sn and operates on these intelligence signals to provide a plurality of spatially modulated and superimposed optical beams, each of which is modulated with a different one of the intelligence signals. This plurality of spatially modulated optical beams is then presented to the input of an optical waveguide 5. The output of the waveguide is coupled to a hologram 6, whereat the plurality of multiplexed optical beams are demultiplexed and reconstructed as individual signals at the individual photosensors of a photosensor array 7, thereby recovering the original plurality of intelligence signals S1-Sn. In FIG. 1 of this patent the optical waveguide comprises a parabolic index type (self focusing) optical fiber. The holographic system that is located at the input of this optical fiber is of the type shown in U.S. Pat. No. 3,612,641. In this device the known type of input hologram 21 of FIG. 3 is also used at output hologram 6 of FIG. 1 for the purpose of demultiplexing.
U.S. Pat. No. 4,877,297 describes a reconfigurable optical interconnection device that uses a spatial light modulator and a dynamic holographic medium in the form of a photorefractive crystal.
U.S. Pat. No. 3,633,034 describes a time division and space division multiplexed optical communication system that employes an optical fiber, wherein the refractive index of the fiber, as observed in a cross section normal to the fiber's axis, is highest at the axis and gradually decreases toward the outer circumference of the fiber. In this device a plurality of input laser beams are each separately modulated (i.e. by the use of a polarization plane rotating means and an analyzer) by an information signal and then made incident upon the input end surface of the optical fiber, each beam being incident thereon at a different angle. The axial length of the fiber is chosen to be of a critical length, so that the plurality of light beams now leave the exit end surface of the optical fiber at angles that are equal to angles at which the beams were incident on the input end surface of the optical fiber. These spatially separated exit beams are then separately demodulated by the use of a plurality of detectors. In place of a plurality of detectors, a time division multiplexed transmission (FIG. 4) can be separated by the use of an electronic rotary switch or an optical channel distributor (FIGS. 3 and 5).
While the various means contained in the prior art are generally useful for their intended purpose, a need remains for a method and an apparatus which will increase the information carrying capacity of a multimode optical fiber.